1. Field of the Invention
The present invention relates to a semiconductor device having a circuit composed of thin film transistors (hereinafter referred to as TFTs) and a manufacturing method thereof. The present invention relates to, for example, a device represented by a liquid crystal display device (on which a liquid crystal module is mounted) and an electronic device on which such a device is mounted as a part.
Note that the semiconductor device in this specification indicates a device in general, which can function by utilizing a semiconductor characteristic, and an electro-optical device, a light emitting device, a semiconductor circuit, and an electronic device each are the semiconductor devices.
2. Description of the Related Art
In recent years, a technique for constructing a thin film transistor (TFT) using a semiconductor thin film (about several to several hundreds nm in thickness) formed on a substrate having an insulating surface has been noted. The thin film transistor is widely applied to an electronic device such as an IC or an electro-optical device and its development as a switching element of an image display device is particularly demanded.
Conventionally, a liquid crystal display device is known as the image display device. Since a high resolution image is obtained as compared with a passive liquid crystal display device, an active matrix liquid crystal display device is used in many cases. According to the active matrix liquid crystal display device, when pixel electrodes arranged in matrix are driven, a display pattern is formed on a screen. In more detail, when a voltage is applied between a selected pixel electrode and an opposite electrode corresponding to the selected pixel electrode, a liquid crystal layer located between the pixel electrode and the opposite electrode is optically modulated and the optical modulation is recognized as the display pattern by an observer.
The range of use of such an active matrix liquid crystal display device is increased. Demands for a higher resolution, a higher opening ratio, and high reliability are increased along with increase in a screen size. Simultaneously, demands for improvement of productivity and cost reduction are also increased.
Conventionally, when a TFT is manufactured using aluminum as a material of a gate wiring of the above-mentioned TFT, a protrusion such as hillock or a whisker is produced by thermal treatment and an aluminum atom is diffused to a channel forming region. Thus, an operation failure of the TFT and a deterioration of a TFT characteristic are caused. In order to solve this, a metallic material which can be resistant to thermal treatment, typically, a metallic element having a high melting point is used. However, a problem in which a wiring resistance is increased due to increase in a screen size arises, and increase in power consumption and the like are caused.
Therefore, an object of the present invention is to provide a structure of a semiconductor device in which low power consumption is realized even when a screen size is increased, and a manufacturing method thereof
According to the present invention, a gate electrode structure is made to be a laminate structure in which a material film containing mainly TaN or W is used as a first layer for preventing diffusion to a channel forming region, a low resistance material film containing mainly Al or Cu is used as a second layer, and a material film containing mainly Ti is used as a third layer. Thus, a resistance of a wiring is reduced.
According to a structure of the present invention disclosed in this specification, a semiconductor device including a TFT which is composed of a semiconductor layer formed on an insulating surface, an insulating film formed on the semiconductor layer, and a gate electrode formed on the insulating film is characterized by comprising: a pixel portion including a first n-channel TFT having a source wiring made of the same material as the gate electrode; a driver circuit including a circuit which is composed of a second n-channel TFT and a third n-channel TFT; and a terminal portion made of the same material as the gate electrode.
In the above-mentioned structure, the gate electrode is characterized by having a laminate structure of a material film containing mainly TaN (a first layer), a material film containing mainly Al (a second layer), and a material film containing mainly Ti (a third layer). Also, the gate electrode is characterized by having a laminate structure of a material film containing mainly W (a first layer), a material film containing mainly Al (a second layer), and a material film containing mainly Ti (a third layer).
According to such agate electrode structure, when an ICP (inductively coupled plasma) etching method is used, end portions of the gate electrode can be formed into a taper shape. Note that a taper angle in this specification indicates an angle formed by a horizontal surface and a side surface of a material layer. Also, in this specification, a side surface having the taper angle is called a taper shape and a portion having the taper shape is called a taper portion.
Also, in the above-mentioned structure, the present invention is characterized in that the second n-channel TFT and the third n-channel TFT compose an EEMOS circuit or an EDMOS circuit. The driver circuit of the present invention is made from an NMOS circuit composed of only n-channel TFTs, and the TFTs of the pixel portion are also composed of n-channel TFTs. Thus, a process is simplified. A general driver circuit is designed based on a CMOS circuit composed of an n-channel semiconductor element and a p-channel semiconductor element, which are complementally combined with each other. However, according to the present invention, the driver circuit is composed of a combination of only n-channel TFTs.
Further, in order to achieve the above-mentioned structure, according to a structure of the present invention, there is provided a method of manufacturing a semiconductor device including a driver circuit, a pixel portion, and a terminal portion, which are located on an insulating surface, the method comprising the steps of:
forming a semiconductor layer on the insulating surface;
forming a first insulating film on the semiconductor layer;
forming a gate electrode, a source wiring of the pixel portion, and an electrode of the terminal portion on the first insulating film;
adding an impurity element for providing an n-type to the semiconductor layer using the gate electrode as a mask to form an n-type impurity region;
etching the gate electrode to form a taper portion;
forming a second insulating film which covers the source wiring of the pixel portion and the terminal portion; and
forming a gate wiring and a source wiring of the driver circuit on the second insulating film.
In the above-mentioned structure, it is characterized in that, in the step of forming the gate electrode, the source wiring of the pixel portion, and the electrode of the terminal portion, a material film containing mainly TaN, a material film containing mainly Al, and a material film containing mainly Ti are formed to be laminated, and then etched using a mask to form the gate electrode, the source wiring of the pixel portion, and the electrode of the terminal portion. Also, in the above-mentioned structure, it is characterized in that, in the step of forming the gate electrode, the source wiring of the pixel portion, and the electrode of the terminal portion, a material film containing mainly W, a material film containing mainly Al, and a material film containing mainly Ti are formed to be laminated, and then etched using a mask to form the gate electrode, the source wiring of the pixel portion, and the electrode of the terminal portion.
Also, according to the present invention, a liquid crystal display device having the pixel portion and the driver circuit as described in the above-mentioned structure or a light emitting device with an OLED having the pixel portion and the driver circuit as described in the above-mentioned structure can be manufactured.
Also, according to the present invention, since a step of manufacturing a p-channel TFT is omitted, a manufacturing step of a liquid crystal display device or a light emitting device is simplified and a manufacturing cost can be reduced.